Gas clathrate hydrates are nonstoichiometric crystalline solids formed from the reaction of water and gas under certain conditions of relatively high pressure and low temperature. (See, e.g. Sloan et al., Clathrate Hydrates of Natural Gases. 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, Fla., 2008) Some efforts have been made in developing natural gas hydrate (NGH) off-shore and on land supply chains to efficiently transport natural gas hydrates to sites for re-gasification and eventual consumption. (See, e.g. Gudmundsson et al., Transport of Natural Gas as Frozen Hydrate. International Society of Offshore and Polar Engineers, 1995; Nogami et al., Development Of Natural Gas Supply Chain By Means Of Natural Gas Hydrate (NGH). International Petroleum Technology Conference: 2008; Rehder et al., Energies 2012; 5(7):2499-2523; Tamsilian et al., J. Dispersion Sci. Technol. 2013; 34(2):259-267). It would be desirable to find safe, efficient and environmentally friendly ways of obtaining stranded gas, such as natural gas or hydrogen, from underwater environments and safely transporting it for consumption. The research literature often focuses on the formation of methane clathrate hydrates (MCH) since methane is the primary component of natural gas. Yet, depending on the origin, the range of possible gas composition may range from nearly pure methane to complex mixtures, rich in heavier volatile hydrocarbons. The processing described herein more generally refers to formation of natural gas clathrate hydrate (NGCH) where the starting natural gas may be about 85% methane, 10% ethane and 5% propane by mass. The process may be easily adapted to other compositions. One key aspect is that the formation of NGCH from natural gas mixtures generally requires less hydrostatic pressure at the same temperature than the formation of MCH from pure methane (FIG. 1). Also, other less typical or artificially blended hydrocarbon gas mixtures, involving less methane and more ethane, propane, butane and iso-pentane, etc., also form solid clathrate hydrates and the higher the proportion of these higher molecular weight gas components, the lower the pressure required. However, the temperature range for clathrate formation becomes much narrower and in the case where the formation from pure butane requires water temperature below 4° C. (FIG. 2) for the range of temperatures and pressures required for clathrate hydrate formation from these pure hydrocarbon gasses.
Fundamental to MCH and NGCH processing is to take advantage of water temperatures of 6° C. or lower at the ocean floor below 800 meters depth, where the corresponding pressure is 8.24 MPa (˜1200 psi) (FIG. 1). At such pressures and depths, methane gas when bubbled into sea water very quickly forms small MCH spheres and flakes as does natural gas (See, e.g. Romer et al., Journal of Geophysical Research: Oceans 2012; 117; Warzinski et al., Geophysical Research Letters 2014; 41(19):2014GL061665). Temperature profiles in the ocean vary at various global locations. The Gulf of Mexico tends to be a few degrees warmer than the North Sea so the depth needed to rapidly form MCH must be greater in the case of the former for a comparable temperature range. Some additional things to add here is that natural gas is considered an undesirable component to deal with when exploring deep ocean petroleum reserves. Stranded gas is sometimes flared or pumped back into the ground in order to prevent flaring the gas. The gas content is one criteria for selecting which reserves to produce and too high a gas contact can preclude development.
Natural gas is considered an undesirable component to deal with when exploring deep ocean petroleum reserves. Stranded gas is sometimes flared or pumped back into the ground. The gas content is one criteria for selecting which reserves to produce and too high a gas content can preclude development. There are earlier and ongoing efforts towards converting natural gas to hydrates in plants on land and on the sea surface. (See, e.g. Nakai, “Development of Natural Gas Hydrate (NGH) Supply Chain,” World Gas Conference 2012, Kuala Lampur; Rehder et al., Energies 2012; 5(7):2499-2523). The exothermic nature of the conversion process and the requirement for high pressure and temperature make the process hazardous, complicated and expensive. It is desirable to provide safe and effective methods for converting natural gas (mostly methane) to solid clathrate hydrates (fire ice) by performing the processing deep in the ocean where the in situ conditions (i.e., the hydrostatic pressure is high and water is cold) is conducive to the formation of clathrates. Doing so would, in turn, provide efficient and environmentally friendly ways of obtaining natural gas from underwater environments and safely transporting it for consumption. Further, because of the exothermic nature of the conversion process, the heat released from the process provides an added valuable resource for exploring deep ocean reserves of natural gas clathrates.